Common shape memory alloy (also called SMA) materials such as Nitinol can be shape-set by heating to an annealing temperature while constrained to a shape, and then cooling. For example, a helix may be formed from a straight TiNi wire by winding the wire on a mandrel, securing the ends, heating to 550° C., and cooling. The rates of heating and cooling are not critical in this ‘shape-setting’ process for TiNi, although special characteristics are achieved by holding the temperature constant at specific temperatures.
It is a general object of the invention to provide methods for the shape setting fabrication of single crystal shape memory alloys (also called “hyperelastic”), such as CuAlNi, CuAlMn, CuAlBe, CuAlNb and others, and to provide devices made by such methods.
A further object of this invention is to provide new and improved devices made of hyperelastic single crystal SMA by novel methods of shape-setting.
In particular, it is the object of the invention to provide new and improved SMA wires which may be used for orthodontic archwires.
In orthodontics, a standard procedure to correct malocclusions is to attach individual teeth to a flexible component called an archwire. These archwires are generally of a simple curved shape as illustrated in the FIG. 3. Examples of stainless steel and titanium-nickel dental arches are described and illustrated on the website ORMCO.com. Dental arches are commonly made of wire of round or rectangular cross-section. Round wires range in wire diameter from 0.013 to 0.026 inches, while rectangular cross-section arches may be as small as 0.016 by 0.016 inches or a large as 0.020 by 0.030 inches.
Millions of orthodontic archwires are sold in the U.S. each year. Titanium-nickel-based alloys have taken market share because they have elasticity superior to that of stainless steel. Increased elasticity enables the orthodontist to move teeth to their desired position with fewer adjustments and fewer replacements of the archwire.
A useful application of hyperelastic wire is in dental arches. For example, archwires made of hyperelastic alloy may have four important advantages over TiNi-based wires: (1) recoverable strain is increased from 3-5 percent to 9-12 percent; (2) force exerted by a hyperelastic wire is less than the force exerted by a TiNi wire of comparable thickness; (3) the force is constant over a displacement of 9 percent strain; and (4) the return force is nearly equal to the displacing force because the mechanical hysteresis is very small. Because of these advantages, hyperelastic archwires may potentially replace titanium-nickel-based archwires in a large percentage of orthodontic procedures. At least one archwire manufacturer has been seeking a new material with these advantages. Unfortunately, typical methods of fabricating such wires may not work for the fabrication of hyperelastic materials.
For convenience of use, straight wires are typically formed into an arch shape that approximately conforms to the shape of the jaw of the patient, a process known as shape-setting. TiNi based alloy wires are shape-set by winding on a mandrel having the desired arch shape, annealing by heating in a furnace to 500° C. or higher, and allowing the shaped wires to cool. This shape-setting process may take an hour or more.
The process described for shape-setting TiNi based wire cannot be used for shape-setting hyperelastic wire. Hyperelastic wire, typically single crystal, is not thermodynamically stable. At elevated temperatures of several hundred degrees, one or more of the components (especially Al) gradually forms precipitates. These precipitates remove the element from the crystal lattice, effectively changing the composition and hence the transition temperature of the alloy. A wire heated to 500° C. and cooled over a period of several minutes has no shape memory and no superelasticity.
This property, the precipitation of Al at high temperature, can be avoided if the wire is heated and cooled rapidly, for example in a few seconds. It is impractical to quickly heat and cool a mandrel of mass sufficient to impart the desired shape. This has led us to the invention of a novel process for shape-setting hyperelastic wires for orthodontic arches, as described herein.
The present invention accomplishes, using a novel method, the purpose of imparting an arch shape to a straight hyperelastic wire while preserving its elasticity and transition temperature.